The present invention relates to rate gyroscopes such as those used in aircraft and missile guidance systems. An example of such a gyroscope is disclosed in the applicant's earlier British Pat. No. 2079954B. In this dual axis rate gyroscope a rotor is mounted so as to be free to tilt about two mutually perpendicular axes in a plane normal to its axis of rotation. Such a rate gyroscope has the characteristic feature that a torque is applied to the rotor to precess it in the direction in which the aircraft or missile is turning. Fixed coils positioned towards the periphery of the rotor act as angle pick-offs sensing the tilt of the rotor. The coils also provide a torque to counter the tilting under the control of a servo system driven by the outputs of the angle pick-off. The dynamics of the system are such that the applied torque along one axis is proportional to the angular rate applied about the other axis.
In practice the performance of gyroscopes of this type is limited by various factors. The angle pick-offs are linear over a finite range only and so limit the effective dynamic range of the system in situations where the applied angular accelerations are high. The design of the control loop is compromised by the need to suppress rotor torque noise at rotor-speed frequencies, the need to provide nutation control and to ensure a stable control loop. Such limitations combine significantly to restrict the rate measurement bandwidth of conventional dual-axis rate gyroscopes.
It is known to use linear accelerometers in displacement gyroscope systems. Once such system is disclosed in U.S. patent application Ser. No. 3,587,330. In such displacement gyroscopes torques are applied to their rotors to maintain them absolutely vertical with respect to the earth's gravitational field. The value of the angle between the gyroscope and its base plate are then read off by angle pick-offs to provide the output signal. The accelerometers are used simply to provide the long term stability of the gyroscope by sensing the deviation from the vertical and torquing the gyroscope accordingly. In a rate gyroscope by contrast, far from being maintained at the vertical the rotor of the gyroscope is torqued each time the vehicle moves so that it precesses in the direction in which the vehicle is turning. The dynamics of such a rate gyroscope system are therefore entirely different from the displacement gyroscopes known in the art.